The North America Numbering Plan (NANP) specifies a 10-digit format for directory numbers, which are more commonly referred to as telephone numbers. The NANP is an integrated plan for assigning directory numbers throughout the United States (including Puerto Rico and the U.S. Virgin Islands), Canada, Bermuda, Anguilla, Antigua, Bahamas, Barbados, British Virgin Islands, Cayamn Islands, Dominica, Dominican Republic, Grenada, Jamacia, Monteserrat, St. Kitts & Nevis, St. Lucia, St. Vincent, Turks & Cicos Islands, and Trinidad & Tobago.
A directory number is typically designated as NPA-NXX-XXXX, where the first three digits are the Numbering Plan Area (NPA) code, which is more commonly referred to as the area code. The next seven digits, designated as NXX-XXXX, are a terminating destination code. The first three digits of the terminating destination code, designated as NXX, are the central office code, also called the prefix or exchange. The last four digits of the terminating destination code, designated as XXXX, are a line, station, or subscriber code. The digits designated as "N" may be any number 2 through 9 and the digits designated as "X" may be any number 0 through 9.
In the NANP as originally implemented, area codes were limited to the format NO/1X. That is, the first digit of the area code could be any number 2 through 9, the second digit of the area code could be either a 1 or a 0, and the third digit of the area code could be any number 0 through 9. This NO/1X format, which provided for a maximum of 160 area codes, was changed as of Jan. 1, 1995 due to a projected shortage of area codes. The new area code format, NXX, allows the second digit of the area code to be any number 0 through 9. The new area code format increased the number of permissible area codes by 640, to a total of 800 available area codes. For more information relating to the NANP, see the 1996 annual status report to the industry on Numbering Plan Area (NPA) codes, published on Jan. 30, 1996 as Bellcore Letter IL-96/01-016, which is incorporated by reference.
This increase in the number of available area codes has allowed many geographic areas to add new area codes in anticipation of increased directory number requirements. Although some subscribers may find the assignment of new area codes to be somewhat inconvenient, the administrators of the NANP have no choice but to add a new area code when nearly all of the directory numbers available under an existing area code have been assigned. Moreover, adding area codes provides local telephone service providers with additional flexibility for implementing billing and dialing plan alternatives for their subscribers. The number of area codes in use in North America, therefore, will likely continue to increase for the foreseeable future.
A distinction should be understood between the numbering plan, which is described above, and what is referred to as a dialing plan. The number plan, which refers to the assignment of directory numbers, is consistent throughout the NANP area and conforms to international numbering standards. A dialing plan, on the other hand, refers to the sequence of digits that a user must dial to place a telephone call from a particular originating station. For many years, only two dialing plans were required in the NANP: a non-toll or local telephone call (i.e., a telephone call to a directory number within the originating station's home area code) was placed by dialing the desired 7-digit terminating destination code; and an "InterLata" long distance or toll telephone call was placed using an 11-digit dialing plan (i.e., by dialing the digit one plus the desired 10-digit directory number). But in recent years, a third, 10-digit dialing plan has been adopted in many locations. This 10-digit dialing plan may not applied consistently across the NANP area and, therefore, can cause confusion in the telephone-using population.
The use of 10-digit dialing plans are not consistent across the NANP area because state or other local regulatory agencies generally determine dialing plans at the local level. As a result, demographic anomalies are often reflected in dialing plans that vary somewhat from location to location. In particular, localities that are close to or straddle area code boundaries may mandate 7-digit dialing zones that encompass portions of two or more area codes. This occurs frequently because area code boundaries often follow state or other political boundaries whereas communities often develop at the confluence of several political subdivisions.
For these multi-area-code communities, regulators may require a 7-digit dialing zone that includes parts of several area codes. Local regulatory requirements may also produce a dialing plan that includes a 7-digit dialing zone and a 10-digit dialing zone within the same area code. In this situation, therefore, a dialed area code is not always sufficient to determine the appropriate dialing plan. Rather, subscribers must learn or look-up the dialing plan on a per-directory-number basis.
Furthermore, area codes are often assigned by splitting a region into two area codes that serve two distinct, usually adjacent, geographic regions.
Alternatively, a new area code may be assigned by overlaying a new area code over an existing area code in the same geographic region. When this happens, a large number of terminating destinations that were formerly in a subscriber's home area code may suddenly become part of a foreign area code. Although the local telephone service provider typically allows non-toll telephone calls between these two area codes, across-area-code telephone calls typically require a 10-digit dialing plan, whereas within-area-code telephone calls typically require a 7-digit dialing plan.
For example, the 404 area code for the Atlanta, Ga. region was recently divided into a 404 area code and a 770 area code. As a result, a 7-digit dialing plan may be used to place a telephone call from a 404 directory number to a 404 directory number, whereas a 10-digit or 11-digit dialing plan must be used to place a telephone call from a 404 directory number to a 770 directory number. An InterLata telephone call from either the 404 area code or the 770 area code requires an 11-digit dialing plan including the digit one plus the desired 10-digit directory number. It should be noted that a toll telephone call may be InterLata or IntraLata. For instance, an IntraLata telephone call from the 404 area code to the 770 area code could be a non-toll (i.e., 10-digit) or a toll (i.e., 11-digit) telephone call depending on the distance between the originating and terminating stations.
Generally stated, the continuing increase in the number of area codes will likely be accompanied by a continuing increase in new 10-digit dialing plans. As a result, a significant number of long-time telephone service subscribers, many of whom are used to using only two dialing plans, will have to learn to use a third, 10-digit dialing plan for the first time. This will lead to increased levels of confusion in the telephone-using population and a corresponding increase in the number of dialing-plan-related service calls that must be handled by local telephone service providers.
To make matters worse, many telephone users have come to rely on caller ID call-back devices to place telephone calls to received directory numbers. These devices typically receive the full 10-digit originating directory number, without any preceding dialed digits, regardless of the dialing plan used to place the in-coming telephone call. Therefore, a caller ID call-back device must be configured to dial the correct dialing plan to place an out-going telephone call to the received caller ID directory number. This typically requires the device to be programmed to accommodate the various dialing plans for a particular location. But pre-programming each caller ID call-back device to accommodate a particular set of dialing plans is complicated by the fact that dialing plans depend on the location of use and can change in an unpredictable manner. Therefore, some prior art caller ID call-back devices are user-programmable.
For example, caller ID call-back devices having two dialing plan selection devices--one for a local (e.g., 7-digit or 10-digit dialing plan) and a second for a long distance (e.g., 11-digit dialing plan)--have been in use for a number of years. These devices may typically be programmed with area codes that require a 7-digit dialing plan and/or a 10-digit dialing plan for local calls, in order to accommodate 7-digit, 10-digit, and 11-digit dialing plans using only two dialing plan selection devices. This allows these devices to be programmed to operate properly in many locations. But because these devices rely on area codes to determine dialing plans, they cannot be programmed to operate properly for all directory numbers in locations in which more than one dialing plan is required to place telephone calls to directory numbers within the same area code.
Moreover, some users will inevitably fail to program their caller ID call-back devices properly. For example, a user may purchase a caller ID call-back device and initially program the device to operate in a particular location. The user may later move the caller ID call-back device to a new location. In many cases, the caller ID call-back device must be reprogrammed to operate properly in the new location. If the user has lost or discarded the instruction manual, the user may have to place a service call to the local telephone service provider or the manufacturer of the caller ID call-back device to determine how to reprogram the device. Therefore, disseminating a large number of caller ID call-back devices with two dialing plan selection devices can be expected to cause a significant number of service calls.
There is, therefore, a need for a caller ID call-back device that is intuitively easy to understand. There is a further need for a caller ID call-back device that does not have to be programmed to properly dial selected directory numbers using three different dialing plans.